Engine exhaust emissions are becoming increasingly important for engine manufacturers. Governments and regulatory agencies are enforcing ever more stringent emissions standards for many types of on-highway and off-highway vehicles. The amount of pollutants in an exhaust flow emitted from the vehicle's engine must be regulated depending on the type, size, and/or class of engine. Manufacturers must develop new technologies to meet these standards while providing high-performance, cost-effective equipment to consumers.
One method implemented by engine manufacturers to comply with the regulation of exhaust flow pollutants is the use of a selective catalytic reduction (“SCR”) catalyst to clean nitrogen oxides (“NOx”) from the engine exhaust flow. An SCR system works by releasing a reductant, such as ammonia (“NH3”), into the engine exhaust flow in the presence of a catalyst. The NH3 may be stored on the surface coating of the catalyst where it reacts with the NOx in the exhaust flow to create environmentally friendly products, such as nitrogen gas (“N2”) and water (“H2O”). The chemical reactions of the SCR process can be represented by:NH3(g)NH3(ads);  (1)4NH3(ads)+4NO+O2→4N2+6H2O;  (2)4NH3(ads)+2NO+2NO2→4N2+6H2O;  (3)8NH3(ads)+6NO2→7N2+12H2O;  (4)4NH3(ads)+3O2→2N2+6H2O.  (5)Reaction (1) describes the ammonia adsorption/desorption from the catalyst, Reactions (2)-(4) are “DeNOx” reactions that describe the reaction between the reductant and the NOx in the presence of the catalyst, and Reaction (5) describes the oxidation of the ammonia.
In general, manufactures seek to maximize the amount of NOx in the exhaust flow converted to H2O and N2. To achieve this, the amount of NH3 stored on the catalyst's surface may be increased. However, NH3 may also be desorbed from the catalyst and carried by the exhaust flow downstream of the catalyst to a location where the NH3 is released into the atmosphere (i.e., slip). NH3 slip is undesirable because the unreacted NH3 is released into the atmosphere and wasted. The NH3 desorption rate is strongly dependent on the catalyst's temperature. As the temperature of the catalyst increases, the desorption rate of NH3 from the catalyst's surface increases exponentially.
Unlike industrial or stationary SCR applications where engines or turbines generally operate at steady state conditions, mobile SCR systems used for on-highway trucks and off-road machines are subject to transient engine speeds and loads. The transient engine speeds and loads lead to a time varying exhaust temperature, and thus a time varying catalyst desorption rate. Automatic control has been used as one method of attempting to handle transient changes in the exhaust gas temperature, while still maintaining a good NOx conversion and avoiding slip.
One method of controlling an SCR process is described in U.S. Pat. No. 7,200,990 (the '990 patent) issued to Gabrielsson et al. on Apr. 10, 2007. Specifically, the '990 patent discloses a method for controlling injection of a reductant into an NOx containing exhaust gas stream from a combustion engine. Step 1 of the method is a stoichiometric calculation of the amount of NOx created by the combustion. The calculation is based on measurement of air to combustion, measurement of O2 content in the exhaust gas, and NOx content. Step 2 calculates the maximum possible or wanted NOx conversion based on the same three measurements as step 1 plus measurement of a temperature of the exhaust gas inlet and outlet of the catalyst (i.e., upstream and downstream of the catalyst, respectively). Results from steps 1 and 2 are used in step 3 to obtain the theoretically required amount of urea solution to be injected at a certain moment.
This theoretical amount is further adjusted in an event based filter, step 4, based on measurement of exhaust gas temperature inlet of the catalyst, air flow measurement, O2 measurement, and on determination of exhaust gas flow as in step 2 in order to avoid leakage of ammonia or NOx during transient conditions. The amount of ammonia adsorbed on the catalyst surface changes especially with changes in exhaust gas flow and temperature. The filter takes into consideration the historical data of the catalyst in order to foresee the NH3 adsorption/desorption capacity of the catalyst. If the conditions of the catalyst are such that a large desorption of ammonia can occur, then part of the calculated (step 3) urea injection is retained and stored in the memory of the injection algorithm. On the other hand, if the conditions are favorable for the adsorption of NH3 on the catalyst, then the actual urea injection can be increased until the amount of urea as stored in the memory is used up.
Although the '990 patent may outline a method of controlling injection of a reductant based on the temperature of the exhaust gas inlet and outlet of the catalyst, the results produced by the control system may be suboptimal. For example, the engine may create favorable conditions, thus causing the control system to store a larger amount of urea. However, a sudden increase in engine load and/or speed may create a sharp increase in the temperature of the exhaust gas. This sharp increase in the exhaust gas temperature may heat the catalyst and significantly increase desorption of the stored urea. Due to the rapid speed at which the heated exhaust gas may travel and a time lag created by the injection, absorption, and desorption processes, even immediately modifying the amount of injected urea upon sensing a temperature change upstream of the catalyst may not be sufficient to prevent the slip of the already stored urea.
The present disclosure is directed at overcoming one or more of the problems set forth above.